1. Field of the Invention
The present invention relates to a position sensible liquid crystal display (PSLCD) device, and more particularly, to a position sensible liquid crystal display device having the capability of sensing the precise position of a stylus on the display.
2. Discussion of the Related Art
In general, as shown in FIG. 1, a liquid crystal display (LCD) device includes an upper plate 3, a lower plate 1, and a liquid crystal sealed between the upper and lower plates. The upper plate 3 has a common electrode 6, a layer of black matrix 4, and a layer of R (red), G (green), and B (blue) color filters 5 that filter light to generate colors. The lower plate has a plurality of data lines and scanning lines arranged at right angles and at fixed intervals to form a matrix of pixel regions therebetween. Each of the pixel regions has a thin film transistor and a pixel electrode. More particularly, lower plate 1 has thin film transistors 2 disposed thereon at fixed intervals, each with a gate electrode G (corresponding to a scanning line), a source electrode S, and a drain electrode D (corresponding to a data line). Each of the pixel regions has a pixel electrode 2a connected to the drain electrode D of the thin film transistor 2. Black matrix 4 on the upper plate 3 blocks light in sections other than pixel electrodes 2a, which corresponds to the R, G, and B color filters 5. Upon selective application of driving signals from external driving circuits to the scanning lines and the data lines, the LCD device displays an image. Recently, much effort has gone into providing the LCD with a position sensor input device so that the LCD can be used like a note book. That is, a position sensible LCD device is used with a position sensor input device to display letters or graphics written with a stylus on the LCD.
FIGS. 2A and 2B illustrate layers in a position sensible LCD device, wherein FIG. 2A illustrates a PSLCD device having an opaque position sensor input device and FIG. 2B illustrates a PSLCD device having a transparent position sensor input device.
The PSLCD device having an opaque position sensor input device includes a PSD (Position Sensor input Device) panel 21, a back light layer 22, an LCD panel 23, and a protective layer 24 arranged in succession. The PSLCD device having a transparent position sensor input device includes a back light layer 22, an LCD panel 23, and a PSD panel 21 arranged in succession.
The operation of the aforementioned PSLCD devices will be explained.
FIG. 3 is a block diagram of the modules of a PSLCD system.
A driving pulse circuit 12 alternately provides driving pulses to the PSD panel 21 in X-, and Y-axis directions. LCD driving circuit 13 provides a driving signal to the LCD in PSLCD 11. Personal computer 14 controls the LCD driving circuit 13. A stylus 15 is sensed according to capacitive coupling occurring in the PSD. A PSD data processing circuit 16 processes a position signal of the stylus 15 to provide position data. Microcomputer 17 controls the driving pulse generating circuit 12 and transfers the position data from the PSD data processing circuit 16 to the personal computer 14. Therefore, the personal computer 14 controls the LCD driving circuit 13 so that the personal computer 14 can display the pixel on which the stylus is placed.
The operation principle of the aforementioned PSLCD module will now be explained in more detail.
Under the control of the microcomputer 17, the driving pulse generating circuit 12 provides driving pulses to X-, and Y-axis of the PSD alternately for sensing the present position of the stylus 15. The stylus 15 senses a position signal in a potential distribution on the PSD using capacitive coupling and provides the position signal to the PSD data processing circuit 16. The PSD data processing circuit 16 receives the present X- and Y-axis coordinate data of the stylus and converts the coordinate data into digitized position data. Upon reception of the position data from PSD data processing circuit 16, the microcomputer 17 analyzes the position data to calculate the present position of the stylus 15 and updates personal computer 14 accordingly. Thus, the pixel of the LCD on which the stylus 15 is placed is displayed under the control of the microcomputer 17.
A conventional PSLCD will be explained with reference to the attached drawing.
FIG. 4 illustrates a perspective view of a disassembled conventional transparent PSLCD.
Referring to FIG. 4, the transparent PSLCD includes an LCD panel 31, a PSD 32 on the LCD panel 31, and a protective layer 33. The LCD panel 31 is the conventional LCD shown in FIG. 1. The PSD 32 is adapted to sense a position of the stylus according to a potential distribution of a driving AC signal, and the protective layer 33 is provided for protecting the PSD from the stylus.
FIG. 5 illustrates a plane view of a configuration of a conventional PSD.
Referring to FIG. 5, the conventional PSD includes a tablet 41 (sometimes called "digitizer"), first, second, third and fourth ITO layers 42, 42a, 43 and 43a on four sides of the tablet 41. The tablet 41 has a plurality of grids 44 spaced at fixed intervals in X- and Y-axis directions. The grids 44 in the X- and Y-axis directions are formed with transparent electrodes (ITO) having uniform internal resistances and are isolated from one another. Each of the first and second ITO layers 42 and 42a applies a driving voltage in the X-axis direction for sensing a position of the stylus. Each of the third and fourth ITO layers 43 and 43a applies a driving voltage in Y-axis direction for sensing a position of the stylus. Each of the first, second, third and fourth ITO layers 42, 42a, 43 and 43a has a switch for selective application of a grounding voltage or a source voltage to the grids in X- and Y-axis directions of the tablet 41. The switches in the first and second ITO layers 42 and 42a are denoted XS1 and XS2, respectively, and the switches in the third and fourth ITO layers 43 and 43a are denoted YS1 and YS2, respectively. In the conventional PSD, the first and second ITO layers 42 and 42a and the third and fourth ITO layers 43 and 43a are alternately applied a source voltage Vcc from driving pulse generating circuit 12.
When the switches XS1 and XS2 are switched on, the switches YS1 and YS2 are switched off. If the source voltage is applied to first ITO layer 42 and a ground voltage is applied to the second ITO layer 42a, the potential of the grids 44 in the tablet 41 gradually decrease between the first ITO layer 42 and to the second ITO layer 42a. This is because, although the X- and Y-axes grids have uniform resistances throughout the tablet, if the source voltage Vcc is applied from one side, there will be a potential difference between a grid 44 next to a source voltage input terminal and a grid 44 farthest from the source voltage input terminal because of a potential difference due to distance. By using the characteristic of the grids having the potential difference, it is possible to detect a position of a particular point if a source voltage and a grounding voltage are selectively applied to the X- and Y-axes directions. That is, after selective application of a source voltage and a grounding voltage, potentials in the X- and Y-axis are detected with a stylus, and the present position of the stylus is derived.
FIG. 6 illustrates a potential distribution on the conventional PSD tablet shown in FIG. 5. A driving voltage is applied to centers of four sides of the conventional PSD. As shown, when a driving voltage is applied to centers of four sides of the conventional PSD, the potential decreases as it goes from the centers to corners of the tablet. This comes from the potential differences caused by distance differences between the grids at the centers which are nearest to voltage input terminals and the grids at the corners which are farthest from the voltage input terminals.
FIG. 7 illustrates a potential distribution on the tablet when a driving voltage is applied to the four comers of the PSD shown in FIG. 5. In general, in order to sense an exact position of the stylus, a linear potential distribution on the tablet is required. However, if a driving voltage is applied to the four corners of the PSD, as shown in shape "A" of FIG. 7, the potential distribution is non-linear, creating non-active regions"a" and "b," which are not detectable with the stylus. The result is that the smaller region"B" is the only region actually detectable with the stylus. The non-linear form of the non-active region implies that the actual active region also has a non-linear form, which means that an exact position sensing is difficult even in the active region in which the stylus can detect a position.
The aforementioned PSLCD device has the following problems.
First, the addition of the PSD region to the LCD device causes the PSLCD device to be bulky and heavy, and have a higher cost.
Second, the connection of the switching device to every grid makes the fabrication process complicated.
Third, the distortion of driving AC signal applied to the PSD causes the potential distribution on the tablet to be non-linear.